Optical communications

ABSTRACT

A method of transmitting a plurality n data streams comprises modulating an optical carrier using differential M-ary phase shift key (DMPSK) signalling in which M=2 n . Advantageously the method comprises using differential quaternary phase shift keying in which n=2. A particular advantage of the method of the present invention is that since the the data is differentially encoded in the form of phase changes rather than absolute phase values this enables the modulated optical carrier to be demolutated using direct detection without requiring a phase-locked local optical oscillator. The invention is particularly applicable to WDM communication systems.

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/GB01/05624, filed 18 Dec. 2001, whichclaims priority to Great Britain Patent Application No. 0031386.6 filedon 21 Dec. 2000 in Great Britain. The contents of the aforementionedapplications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to optical communications and in particular to amethod of modulating and demodulating an optical carrier. Moreespecially the invention concerns a method and apparatus employing suchmodulation and demodulation for use in a wavelength division multiplex(WDM) optical communications system.

With ongoing developments in optically amplified dense wavelengthdivision multiplex (DWDM) optical links as the backbone ofpoint-to-point information transmission and the simultaneous increase inbit rate applied to each wavelength and the simultaneous increase in thenumber of channels, the finite width of the erbium gain window ofconventional erbium-doped optical amplifiers (EDFAs) could become asignificant obstacle to further increases in capacity. ConventionalEDFAs have a 35 nm gain bandwidth which corresponds to a spectral widthof 4.4 THz. System demonstrations of several Tbit/s are already areality and the spectral efficiency, characterised by the value ofbit/s/Hz transmitted, is becoming an important consideration. Currently,high-speed optical transmission employs binary amplitude keying, usingeither non-return-to-zero (NRZ) or return-to-zero (RZ) signallingformats, in which data is transmitted in the form of optical pulseshaving a single symbol level.

In WDM several factors limit the minimum channel spacing for binaryamplitude signalling, and in practice spectral efficiency is limited to˜0.3 bit/s/Hz. Although increasing the per-channel bit rate tends toreduce system equipment, there are several problems that need to beovercome for transmission at bit rates above 10 Gbit/s; these being:

-   -   dispersion management of the optical fiber links, this becomes        increasingly difficult with increased bit rate;    -   Polarisation mode dispersion (PMD) in the optical fiber causes        increased signal degradation;    -   Realisation of electronic components for multiplexing,        de-multiplexing and modulator driving becomes increasingly        difficult.

One technique which has been proposed which allows an improvement ofspectral efficiency is the use of quaternary phase shift keying (QPSK)[S. Yamazaki and K. Emura, (1990) “Feasibility study on QPSK opticalheterodyne detection system”, J. Lightwave Technol., vol. 8, pp.1646-1653]. In optical QPSK the phase of light generated by atransmitter laser is modulated either using a single phase modulator(PM) driven by a four-level electrical signal to generate phase shiftsof 0, p/2, p or 3p/2 representative of the four data states, or usingtwo concatenated phase modulators which generate phase shifts of 0 orp/2 and 0 or 3p/2 respectively. A particular disadvantage of QPSK isthat demodulation requires, at the demodulator, a local laser which isoptically phase-locked to the transmitter laser. Typically this requiresa carrier phase recovery system. For a WDM system a phase-locked laserwill be required for each wavelength channel. It further requiresadaptive polarisation control which, in conjunction with a phaserecovery system, represents a very high degree of complexity.Furthermore, systems that require a coherent local laser are sensitiveto cross-phase modulation (XPM) in the optical fiber induced by theoptical Kerr non-linearity, which severely restricts the application tohigh capacity DWDM transmission.

It has also been proposed to use differential binary phase shift keying(DBPSK) [M. Rohde et al (2000) “Robustness of DPSK direct detectiontransmission format in standard fiber WDM systems”, Electron. Lett.,vol. 36]. In DBPSK data is encoded in the form of phase transitions of 0or p in which the phase value depends upon the phase of the carrierduring the preceding symbol interval. A Mach-Zehnder interferometer witha delay in one arm equal to the symbol interval is used to demodulatethe optical signal. Although DBPSK does not require a phase-locked laserat the receiver it does not provide any significant advantages comparedto conventional amplitude NRZ signalling.

SUMMARY OF THE INVENTION

The present invention has arisen in an endeavour to provide a signallingformat for use in WDM which in part, at least, overcomes the limitationsof the known arrangements.

According to the present invention a method of transmitting a pluralityn data streams comprises modulating an optical carrier usingdifferential M-ary phase shift key (DMPSK) signalling in which M=2^(n).Advantageously the method comprises using differential quaternary phaseshift keying in which n=2. A particular advantage of the method of thepresent invention is that since the data is differentially encoded inthe form of phase changes rather than absolute phase values this enablesthe modulated optical carrier to be demodulated using direct detectionwithout requiring a phase-locked local optical oscillator.

The present invention finds particular application in WDM communicationsystems and according to a second aspect of the invention an opticalwavelength division multiplex communication system is characterised bythe optical wavelength channels being modulated in accordance with theabove method. To improve spectral efficiency by reducing wavelengthchannel separation the WDM system advantageously further comprisesoptical filtering means for optically filtering each DMPSK modulatedwavelength channel before multiplexing them to form a WDM signal andwherein the optical filtering means has a characteristic which isselected such as to minimise inter-symbol interference at the sampletime for each symbol. Preferably the optical filtering means has acharacteristic which is a squared raised-cosine. Advantageously thefiltering means has a transmission characteristic G(f) given by:

${G(f)} = \left\{ \begin{matrix}{{1\mspace{11mu}{for}\mspace{14mu} 0} \leq {{f - f_{0}}} \leq \frac{1 - \alpha}{2\; T}} \\{{\frac{1}{4}\left( {1 - {\sin\left\lbrack {\frac{\pi\; T}{\alpha}\left( {{{f - f_{0}}} - \frac{1}{2\; T}} \right)} \right\rbrack}} \right)^{2}\mspace{14mu}{for}\mspace{14mu}\frac{1 - \alpha}{2\; T}} \leq {{f - f_{0}}} \leq \frac{1 + \alpha}{2\; T}} \\{{0\mspace{14mu}{for}\mspace{14mu}{{f - f_{0}}}} > \frac{1 + \alpha}{2\; T}}\end{matrix} \right.$where T is the sampling period at the demodulator (i.e. 1/line symbolrate), a the excess bandwidth factor of the filter and f₀ the centrefrequency of the WDM channel.

Preferably the filter has an excess bandwidth factor of between 0.2 and0.8. Advantageously the optical filtering means comprises a respectiveoptical filter for each wavelength channel. For ease of integration eachfilter advantageously comprises an optical fiber Bragg grating.

According to a further aspect of the invention a modulator arrangementfor modulating an optical carrier using a quaternary differential phaseshift key in dependence upon two data streams comprises: two MachZehnder interferometers operable to selectively modulate the phase by ±pradians in dependence upon a respective drive voltage; a fixed phaseshifter for applying a phase shift of p/2 and means for generating therespective drive voltages in dependence upon the two data streams andare such that the data streams are encoded in the phase transitionsgenerated by the interferometers.

Advantageously each interferometer has an optical transmission versusdrive voltage characteristic which is cyclic and which has a period ofV_(2p) volts and in which the modulators are operable on a part of saidcharacteristic to give minimum transmission and in which the drivevoltages have values of ±V_(p).

According to yet a further aspect of the invention a modulatorarrangement for modulating an optical carrier using a quaternarydifferential phase shift key (QPSK) in dependence upon two data streamscomprises: phase modulating means operable to selectively modulate thephase of the optical carrier with phase shifts of 0, p/2, p, 3p/2radians in dependence upon the two data streams. Advantageously thephase modulating means comprises optical splitting means for splittingthe optical carrier into two optical signals which are applied to arespective Mach Zehnder modulator which is operable to selectivelymodulate the phase of the optical signal by ±p radians in dependenceupon a respective drive voltage; a fixed phase shifter for applying aphase shift of p/2 to one of said optical signals; and means forcombining said modulated optical signals to produce the DQPSK modulatedsignal.

Preferably with such an arrangement each Mach Zehnder modulator has anoptical transmission versus drive voltage characteristic which is cyclicand which has a period of 2V_(p) and in which the modulators areoperable on a part of said characteristic to give minimum transmissionand in which the drive voltages have values of ±V_(p).

According to a further aspect of the invention a demodulator arrangementfor demodulating a quaternary differential phase shift key (DQPSK)modulated optical signal comprises: optical splitting means forsplitting the DQPSK modulated optical signal, two unbalanced MachZehnder interferometers to which respective parts of the modulatedoptical signal are applied; and a respective balanced optical toelectrical converter connected across the optical outputs of eachinterferometer; wherein each Mach Zehnder interferometer has an opticalpath length difference between its respective arms which correspondswith substantially the symbol period of the modulated signal and inwhich one interferometer is operable to further impart a relative phaseshift of p/2 radians. In a particularly preferred arrangement the MachZehnder interferometers are respectively operable to impart phase shiftsof p/4 and -p/4 radians. Such an arrangement ensures that thedemodulated outputs are symmetrical NRZ signals.

The use of optical filtering as described above is considered inventivein its own right irrespective of the modulation format. Thus accordingto yet a further aspect of the invention a WDM communications system ofa type in which data is transmitted in the form of a plurality ofmodulated optical carriers of selected wavelengths is characterised byoptical filtering means for filtering each modulated carrier beforemultiplexing them to form a WDM signal and wherein the optical filteringmeans has an optical characteristic which is selected such as tominimise inter-symbol interference at the sample time for each symbol.

Preferably the optical filtering means has a characteristic as describedabove. Advantageously the optical filtering means comprises a respectiveoptical filter, preferably a fiber Bragg grating, for each wavelengthchannel.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention can be better understood a modulator anddemodulator arrangement in accordance with the invention will now bedescribed by way of example only with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic representation of an optical modulator arrangementin accordance with the invention;

FIG. 2 is a representation of the functional elements of the pre-codingcircuitry of the modulator arrangement of FIG. 1;

FIG. 3 is a schematic representation of an optical demodulatorarrangement in accordance with the invention;

FIG. 4 is a simulated 20 Gbit/s “eye” diagram for one output of thedemodulator arrangement of FIG. 3;

FIG. 5 is a simulated 20 Gbit/s “eye” diagram for transmission over a600 km optically amplified, dispersion-managed optical fibre link andincludes Signal-ASE (Amplitude Spontaneous Emission) noise;

FIG. 6 shows optical transmission characteristics for (a) a transmitteroptical filter in accordance with the invention and (b) a receiveroptical filter in accordance with the invention;

FIG. 7 is a simulated 20 Gbit/s “eye” diagram of FIG. 4 and furtherincluding optical filtering in accordance with the invention;

FIG. 8 is a simulated 20 Gbit/s “eye” diagram of FIG. 4 and furtherincluding conventional (Butterworth response) optical filtering; and

FIG. 9 shows simulated 20 Gbit/s “eye” diagrams for amplitude modulatedNRZ data with (a) conventional (Butterworth response) optical filteringand (b) optical filtering in accordance with the invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to FIG. 1 there is shown an optical phase shift key modulatorarrangement in accordance with the invention for encoding two 20 Gbit/sNRZ data streams d₁(t), d₂(t) onto a single optical carrier. Typicallythe modulator arrangement would be used as part of a transmitter in aWDM optical communications system with a respective modulatorarrangement for each WDM wavelength channel.

The modulator arrangement comprises a single frequency laser 2, forexample a distributed feedback (DFB) semiconductor laser due to itsstable optical output for a given wavelength, which is operated toproduce an unmodulated optical output of a selected wavelength,typically a WDM wavelength channel. Light from the laser is divided byan optical splitter 4 into two parts and each part is applied to arespective phase modulator 6, 8. Each phase modulator 6, 8 is configuredsuch that it selectively modulates the phase by 0 or p radians independence upon a respective binary (bipolar) NRZ drive voltage V₁(t),V_(Q)(t). In the preferred arrangement illustrated in FIG. 1 the phasemodulators 6, 8 each comprise a Mach-Zehnder electro-optic modulator(MZM) which is fabricated for example in gallium arsenide or lithiumniobate. As is known MZMs are widely used as optical intensitymodulators and have an optical transmission versus drive voltagecharacteristic which is cyclic and is generally raised cosine in nature.The half period of the MZM's characteristic, which is measured in termsof a drive voltage, is defined as V_(p). Within the modulatorarrangement of the present invention each MZM 6, 8 is required tooperate as a phase modulator without substantially affecting theamplitude (intensity) of the optical signal. To achieve this each MZI 6,8 is biased for minimum optical transmission in the absence of a drivevoltage and is driven with a respective drive voltage V₁(t),V_(Q)(t)=±V_(p) to give abrupt phase shifting with a minimum ofamplitude modulation. The two phase modulators 6, 8 have matched delays(phase characteristics).

The optical output from the phase modulator 6 is passed through a fixedphase shifter 10 which applies a fixed phase shift of p/2 such that therelative phase difference between the optical signals passing along thepath containing the modulator 6 and that passing along the pathcontaining the modulator 8 is ±p/2. The optical signals from the fixedphase shifter 10 and phase modulator 8 are recombined by an opticalrecombiner 12, for example a 3 dB coupler, to form an optical phaseshift key (PSK) output 14.

The phase modulator drive voltages, V₁(t), V_(Q)(t) are generated bypre-coding circuitry 16 in dependence upon the two binary data streamsd₁(t), d₂(t). According to the modulator arrangement of the presentinvention the two data streams d₁(t), d₂(t) are differentially encodedsuch that these data are encoded onto the optical signal 14 in the phasetransitions (changes) rather than in the absolute phase value. As aresult it will be appreciated that the optical signal 14 is differentialquaternary phase shift key (DQPSK) encoded. The use of DQPSK to modulatean optical signal is considered inventive in its own right.

The DQPSK optical signal 14 is ideally given by E₀ exp(i?t+?+?_(i)),where ? is the mean optical angular frequency, t is time, θ the carrierphase (NB: as discussed below this is arbitrary in that it is does notneed to be known to subsequently demodulate the signal) and θ_(i) a datadependent phase modulation for the i-th data symbol d_(i). In thegeneral case d_(i)ε{0, 1, . . . M−1} and for quarternary phase shiftkeying M=4, that is the data symbol has four values. The phasemodulation term is given by ?_(i)=?_(i-1)+??_(i)(d_(i)) in which θ_(i-1)is the phase term for the previous data symbol d_(i-1) and Δ?_(i) thechange in phase between the i-₁ and i-th data symbols. The relationshipbetween data symbol d_(i) and phase shift Δ?_(i) for QPSK is tabulatedbelow.

TABLE 1 Values of data d₁(t), d₂(t), data symbol d_(i) and phase changeΔ?_(i)(d_(i)) for DQPSK. d₁(t) d₂(t) d_(i) Δ?_(i)(d_(i)) 0 0 0 0 0 1 1 p/2 1 0 2 p 1 1 3 3p/2It is to be noted that the mapping between data, data symbol and phasechange is just one example and that other mappings can be used. Thepre-coding circuitry 16, a functional representation of which is shownin FIG. 2, is configured such as to produce the appropriate drivevoltages V₁(t), V_(Q)(t) in dependence upon the two data streams d₁(t),d₂(t) according to the relationships:V ₁(i)=V ₁(i−l)cos Δθ(d _(i))−V _(Q)(i−l)sin Δθ(d _(i))  Eq. 1V _(Q)(i)=V ₁(i−l)sin Δθ(d _(i))+V _(Q)(i−l)cos Δθ(d _(i))  Eq. 2.

In FIG. 2 elements having the same functionality are given likereference numerals and respectively comprise an inverter 18, summingelements 20, multiplying elements 22, subtracting elements 24 and delayelements 26. The delay elements 26 have a delay t corresponding tosymbol period which for a 20 Gsymbol/s line rate is approximately 50 ps.

Referring to FIG. 3 there is shown a demodulator arrangement inaccordance with the invention. The demodulator comprises an opticalsplitter 28 for splitting the received DQPSK modulated optical signal 14into two parts which are applied to a respective unbalanced Mach-Zehnderinterferometer (MZI) 30, 32. Typically the MZIs are fabricated ingallium arsenide or lithium niobate. A respective balanced optical toelectrical converter 34, 36 is connected across the optical outputs ofeach MZI 30, 32. Each MZI 30, 32 is unbalanced in that each has a timedelay τ, nominally equal to the symbol period (50 ps for a 20 Gsymbol/sline rate) of the data modulation rate, in one arm 30 a, 32 a relativeto that of the other arm 30 b, 32 b. The time delay τ, is introduced bymaking the optical path length of the two arms different and for ease offabrication is introduced by making the physical length of the MZI's arm30 a, 32 a longer than the other arm 30 b, 32 b. Each MZI 30, 32 isrespectively set to impart a relative phase shift of p/4 and −p/4 by theapplication of an appropriate voltage to electrodes on the shorter arm30 b, 32 b. With balanced detection the output electrical signals x₁(t),y₂(t) from the converters 34, 36 is given by:x ₁(t), y ₂(t)=cos(Δ?(d _(i)))±sin(Δ?(d _(i)))  Eq. 3

For DQPSK where Δθ(d_(i)) takes the possible values {0, p/2, p, 3p/2}the outputs are hence binary (bipolar) signals given by:x ₁(t)=d ₁(t) and y ₂(t)=d ₂(t).

A particular benefit of setting the MZIs 30, 32 to impart relative phaseshifts between their arms of p/4 and −p/4 respectively is that thisresults in the de-modulated signals x₁(t), y₂(t) being symmetricalbipolar NRZ signals. It will be appreciated that the in-phase andquadrature components of the DQPSK signal can also be demodulated usingother relative phase shifts provided there is a difference of p/2between the respective MZIs, though the resulting signals will not besymmetrical bipolar NRZ signals. In the general case, therefore, the MZI30 is set to impart a phase shift f and the MZI 32 set to impart a phaseshift f±p/2.

Referring to FIG. 4 there is shown a simulated “eye” diagram for one ofthe demodulated electrical signals x₁(t) or y₂(t) for a 20 Gbit/s datastream for a communications system incorporating the DQPSK modulator anddemodulator arrangement of the FIGS. 1 and 3. FIG. 5 shows a furthersimulated “eye” diagram for the same system for transmission over a 600km optically amplified, dispersion managed optical fiber link andfurther includes Signal-ASE (Amplitude Spontaneous Emission) noise.

A particular advantage of the signalling format of the present inventionis its improved spectral efficiency. Simulations of a DWDM optical linkcomprising 40 Gbit/s channels with a 50 GHz spacing (0.8 bit/s/Hzspectral efficiency) indicate that such a system is quite viable withoutrequiring polarisation multiplexing/interleaving.

As well as improved spectral efficiency DQPSK offers a number ofadvantages compared to other signalling formats such as binary amplitudeand binary phase shift signalling. Optical DQPSK provides a highertolerance to chromatic dispersion and a higher tolerance to polarisationmode dispersion. A further advantage is that the electrical andopto-electronic components operate with a bandwidth commensurate withhalf the line bit rate. Compared to coherent QPSK, optical DQPSKprovides improved tolerance to cross-phase modulation (XPM) since thesignal is differentially encoded as a phase difference (change) betweensuccessive data bits whilst XPM will in general be common to successivedata bits. Since optical DQPSK does not require a phase-coherent localoscillator for demodulation this eliminates the need for adaptivepolarisation control. Since modulation and demodulation for opticalDQPSK is functionally relatively straightforward it can readily beimplemented using robust and compact electro-optic compact circuits suchas phase modulators, optical couplers, splitters etc which can bereadily integrated in the form of monolithic waveguide devices.

Initial simulations indicate that optical DQPSK is a viable signallingformat for transmission of multiple 40Gb/s DWDM communication channelswith tera bit/s capacity over a single optical fiber it is alsoenvisaged that combined with Raman amplification and forward errorcorrection (FEC), optical DQPSK signalling offers the potential for terabit/s transmission over several thousands of kilometers of opticalfiber.

One of the limiting factors for improving spectral efficiency in WDMsystems is that tight optical filtering of the modulated wavelengthchannels before multiplexing using known optical filters (Butterworth)causes inter-symbol interference (ISI) thereby degrading systemperformance. To overcome this problem the inventor has appreciated thatthe system performance can be further improved by using opticalfiltering if its characteristic is carefully selected such as tominimise ISI at the sampling times t_(s) for the data symbols. For DQPSKthe optical spectrum is close to an ideal “sinc” (sin x/x) function andconsequently the side lobes are quite high compared to other signallingformats. By optically filtering each DQPSK wavelength channel using anoptical filter whose optical characteristic (response) is tailored(shaped) such as to minimise ISI for each symbol at the sampling timesthis reduces spectral overlap of adjacent channels and limits the amountof received noise. Ideally the filter response should achieve pulseshaping such that the received pulse shape p satisfies:

${p\left( {{i\; T} - {k\; T}} \right)} = \left\{ \begin{matrix}{1,{i - k}} \\{0,{i \neq 0}}\end{matrix} \right.$where T is the sampling period (line symbol period=1/line rate).

In an idealised system in which the symbols have a perfect impulseresponse the filter function is required to have a linear phase andfrequency-dependent transmission characteristic G(f) which is a squaredraised cosine which is given by:

$\begin{matrix}{{G(f)} = \left\{ \begin{matrix}{{1\mspace{11mu}{for}\mspace{14mu} 0} \leq {{f - f_{0}}} \leq \frac{1 - \alpha}{2\; T}} \\{{\frac{1}{4}\left( {1 - {\sin\left\lbrack {\frac{\pi\; T}{\alpha}\left( {{{f - f_{0}}} - \frac{1}{2\; T}} \right)} \right\rbrack}} \right)^{2}\mspace{14mu}{for}\mspace{14mu}\frac{1 - \alpha}{2\; T}} \leq {{f - f_{0}}} \leq \frac{1 + \alpha}{2\; T}} \\{{0\mspace{14mu}{for}\mspace{14mu}{{f - f_{0}}}} > \frac{1 + \alpha}{2\; T}}\end{matrix} \right.} & {{Eq}.\mspace{14mu} 4}\end{matrix}$where α is the roll-off factor or excess bandwidth factor and f₀ is thecentre frequency of the WDM channel. Typically the filter is selected tohave an excess bandwidth factor of 0.2 to 0.8. By analogy with RFsignalling the optical filtering in accordance with the presentinvention with hereinafter be referred to as optical Nyquist filtering.

Preferably the optical nyquist filtering function is split betweenoptical Nyquist filters located at the transmitter and receiver. Thetransmitter optical filter is preferably connected to the output of themodulator arrangement such that each wavelength channel is filteredbefore multiplexing the channels and the receiver optical filter locatedafter de-multiplexing of the WDM optical signal and before demodulationof the DQPSK signal.

The optical characteristics of the transmitter and receiver Nyquistfilters are illustrated in FIGS. 6( a) and 6(b). Referring to FIG. 6( b)the receiver Nyquist filter characteristic comprises a squared raisedcosine function, as defined by Eq. 4, with a total bandwidth of 30 GHzthat is centered about the WDM wavelength channel (the WDM wavelengthchannel f₀ is indicated as zero on the frequency axis of FIG. 6), thatis the excess bandwidth factor a is 0.5 for a 20 Gsymbol/s line rate.The transmitter filter characteristic, FIG. 6( a), also essentiallycomprises a squared raised-cosine function but is further modified inthe bandpass region 38

$\left( {0 \leq {{f - f_{0}}} \leq \frac{1 - \alpha}{2\; T}} \right)$to have a 1/“sinc” function to take account of the finite pulse width ofthe transmitted symbols. Preferably the transmitter and receiverNyquists are fabricated in the form of optical fiber Bragg gratings or agrating structure defined within an optical waveguide.

Referring to FIG. 7 this illustrates the resulting effect on thesimulated eye of FIG. 4 of the transmitter and receiver Nyquist filters.It is to be noted that the filters have the effect that the “eye” hasnear zero ISI at the sampling times t_(s). For comparison FIG. 8 showsthe same “eye” for conventional filtering (Butterworth response).

It is further found that the use of optical Nysquist filtering can alsoimprove the system performance for other signalling formats such asbinary amplitude keying as is illustrated with reference to FIGS. 9( a)and (b) which respectively show 20 Gbit/s “eye” diagrams for amplitudeNRZ modulated data with conventional filtering (Butterworth response)and with Nyquist filtering in accordance with the invention. The use ofNyquist filtering, that is optical filtering using a filter having aresponse which is selected to minimise inter-symbol interference foreach symbol at the sampling time, is considered inventive its own rightirrespective of the modulation format.

It will be appreciated that the present invention is not limited to thespecific embodiment described and that variations can be made which arewithin the scope of the invention. Whilst the use of opticaldifferential quaternary phase shift keying (DQPSK) to modulate anoptical carrier is considered inventive in its own right it is alsoenvisaged to use other multi-level (M-ary) DPSK in which M=2^(n) where nis an integer n=2, 3, . . . . Furthermore, whilst the use of MZIs in themodulator arrangement is particularly preferred, other phase modulatorscan be used to phase modulate the optical carrier. These include, forexample, concatenated phase modulators or a single phase modulatordriven with a multi-level drive signal.

1. A method of transmitting a plurality n of binary digital basebanddata streams, comprising: differentially pre-coding the plurality ofbaseband data streams to form a plurality of baseband drive signals,directly driving an optical phase modulator means with the plurality ofdrive signals to generate a differential M-ary (M=2^(n)) phase shiftkey-modulated optical carrier signal; transmitting the differentialM-ary phase shift key-modulated optical carrier signal from said opticalphase modulator means; optically demodulating the differential M-aryphase shift key-modulated optical carrier signal using a firstinterferometer, free of a local oscillator, having first and second armsand first and second outputs, and using a second interferometer, free ofa local oscillator, having first and second arms and first and secondoutputs; the first and the second interferometers each utilizing anoptical delay of substantially one symbol period and imparting arelative phase shift for the optical carrier between its respectivefirst and second arms such that there is a difference of π/2 radiansbetween the relative phase shifts of the first and the secondinterferometers for the optical carrier frequency; performing balanceddetection using a first balanced optical detector at the outputs of thefirst interferometer; and performing balanced detection using a secondbalanced optical detector at the outputs of the second interferometer.2. A wavelength division multiplex (WDM) optical communication systemhaving one or more optical wavelength channels characterized by theoptical wavelength channels being modulated in accordance with themethod of claim
 1. 3. A WMD communications system according to claim 2,further comprising optical filtering means for filtering each of thedifferential M-ary phase shift key modulated wavelength channel beforemultiplexing them to form a WDM signal, and wherein the opticalfiltering means has an optical characteristic which is selected tominimize inter-symbol interference at the sample time for each symbol.4. A WDM communications system according to claim 3, wherein the opticalfiltering means has a characteristic which is substantially a squaredraised-cosine.
 5. A WDM communication system according to claim 3,wherein the filtering means has a transmission characteristic G(f) givenby: ${G(f)} = \left\{ \begin{matrix}{{1\mspace{11mu}{for}\mspace{14mu} 0} \leq {{f - f_{0}}} \leq \frac{1 - \alpha}{2\; T}} \\{{\frac{1}{4}\left( {1 - {\sin\left\lbrack {\frac{\pi\; T}{\alpha}\left( {{{f - f_{0}}} - \frac{1}{2\; T}} \right)} \right\rbrack}} \right)^{2}\mspace{14mu}{for}\mspace{14mu}\frac{1 - \alpha}{2\; T}} \leq {{f - f_{0}}} \leq \frac{1 + \alpha}{2\; T}} \\{{0\mspace{14mu}{for}\mspace{14mu}{{f - f_{0}}}} > \frac{1 + \alpha}{2\; T}}\end{matrix} \right.$ where T is sampling period, α the excess bandwidthfactor of the filter and f₀ the centre frequency of the WDM channel. 6.A WDM communications system according to claim 5, wherein the filter hasan excess bandwidth factor of between 0.2 and 0.8.
 7. A WDMcommunications system according to claim 3, wherein the opticalfiltering means comprises a respective optical filter for eachwavelength channel.
 8. A WDM communications system according to claim 7,wherein each filter comprises an optical fibre Bragg grating.
 9. Amodulator arrangement for modulating an optical carrier using aquaternary differential phase shift key (DQPSK technique) in dependenceupon two binary digital baseband data streams in accordance with claim1, comprising: phase modulating means operable to selectively modulatethe phase of the optical carrier with phase shifts 0, π/2, π, 3π/2radians in dependence upon the two binary digital baseband data streams.10. A modulator arrangement according to claim 9, wherein the phasemodulating means comprises optical splitting means for splitting theoptical carrier into two optical signals which are applied to arespective Mach Zehnder modulator which is operable to selectivelymodulate the phase of the optical signal by ±π radians in dependenceupon a respective drive voltage; a fixed phase shifter for applying aphase shift of π/2 to one said optical signals; and means for combiningsaid modulated optical signals to produce a DQPSK technique modulatedsignal.
 11. A modulator arrangement according to claim 10, wherein eachMach Zehnder modulator has an optical transmission versus drive voltagecharacteristic which is cyclic and which has a period of 2V_(π) and inwhich the modulators are operable on a part of said characteristic togive minimum transmission and in which the drive voltages have values of± V_(π).
 12. A demodulator arrangement for demodulating a quaternarydifferential phase shift key (DQPSK) modulated optical signal that hasbeen modulated in accordance with claim 1, comprising: optical splittingmeans for splitting the DQPSK modulated optical signal, two unbalancedMach Zehnder interferometers to which respective parts of the modulatedoptical signal are applied; and a respective balanced optical toelectrical converter connected across the optical outputs of eachinterferometer; wherein each Mach Zehnder interferometer is operable tofurther impart a relative phase shift between its respective arms suchthat there is a difference of π/2 radians between said relative phaseshifts of the Mach Zehnder interferometers.
 13. A demodulatorarrangement according to claim 12, wherein the Mach Zehnderinterferometers are respectively operable to impart phase shifts of π/4and −π/4 radians.
 14. A method according to claim 1, comprising two ofsaid optical phase modulator means, each of which modulates a part ofthe optical carrier, the two parts of the modulated carrier beingrecombined to generate the differential M-ary phase shift key opticalphase modulated optical signal.
 15. A method according to claim 1,wherein M=4.
 16. A method according to claim 1, wherein the demodulateddifferential M-ary (M=2n) phase shift key optical carrier signalcomprises differential signal components.